get or set document-level meta-data

Get or set document-level meta-data. Document-level meta-data are a special
type of docvars, meant to contain information about documents that
would not be used as a "variable" for analysis. An example could be the
source of the document, or notes pertaining to its transformation, copyright
information, etc.

Document-level meta-data differs from corpus-level meta-data in that the
latter pertains to the collection of texts as a whole, whereas the
document-level version can differ with each document.